설암 수술 후 recurrent laryngeal nerve 손상으로 인한 증상 이해

[문형철]

두경부 암의 증상이해를 위해서는 cranial nerve의 심도깊은 탐구가 필요

예를들어 설암과 주위 임파선 청소술을 시행하다 우측 recurrent laryngeal nerve손상 

목소리 변형(쉰목소리, 고음불가)

삼킴기능저하(dysphagia)

vagus nerve - superior and inferior laryngeal nerve, recurrent laryngeal nerve

The larynx serves multiple functions, including control of respiration, airway protection, coordination of swallowing, and phonation. Several nerves in the larynx control these tasks.

후두는 호흡조절, 기도보호, 삼킴협응작용, 발성기능 등 다양한 역할을 도움. 

몇가지 신경이 후두의 이러한 기능을 조절함.

Vagus nerve

The vagus nerves innervates the larynx. This nerve emerges from the medulla of the brain stem and exits the skull through the jugular foramen. In the medulla, the vagal fibers are connected to 4 nuclei: (1) spinal nucleus of the trigeminal nerve, (2) nucleus of the tractus solitarius, (3) nucleus ambiguus, and (4) dorsal vagal motor nucleus. 

미주신경이 후두신경지배. 

뇌간의 medulla에서 미주신경 섬유는 4개의 핵과 연결됨. 삼차신경의 척수핵, 고립로 핵, nucleus ambiguus(의핵), dorsal vagal motor nucleus..

Two sensory ganglia are associated with the vagus nerve: (1) superior (jugular) ganglion and (2) inferior (nodosum) ganglion. The superior ganglion is located within the jugular fossa of the petrous temporal bone, which, together with the occipital bone, forms the jugular foramen. Exiting the jugular foramen, the vagus nerve enlarges into the inferior (nodosum) ganglion. 

In the neck, the vagus nerve lies posterior to, and in a groove between, the internal jugular vein and the internal carotid artery (see the image below).

Anatomy of the superior laryngeal nerve (SLN) recurrent laryngeal nerve (RLN), posterior view. 

Superior laryngeal nerve

The superior laryngeal nerve (SLN) branches from the main trunk of the vagus high in the neck. It descends in the neck adjacent to the pharynx, medial to the carotid sheath. The SLN divides into the internal and external branches approximately 2-3 cm superior to the superior pole of the thyroid (see the images below).

Anatomy of the superior laryngeal nerve (SLN), left lateral view. 

Right lateral view of the superior and recurrent laryngeal nerves. 

Internal branch of the SLN

General sensation, including pain, touch, and temperature for the tissue superior to the vocal folds, accompany the visceral sensory axons and run in the internal branch of the SLN. Secretomotor fibers are also in the internal laryngeal branch of the SLN. The internal branch of the SLN exits the larynx and pharynx through a foramen in the posteroinferior portion of the thyrohyoid membrane (see the first image above).

Also passing through this foramen is the superior laryngeal artery. The nerve provides sensation of the base of tongue, both surfaces of epiglottis, the aryepiglottic folds, and the vestibule of the larynx to the level of the vocal folds. 

The internal branch of the SLN is then united with the external branch of the SLN to ascend in the neck to join the rest of the vagus before it reaches the inferior vagal ganglion. 

External branch of the SLN

The branchial motor axons in the external branch of the SLN supply the inferior constrictor muscles and the cricothyroid muscles. The external branch descends to the region of the superior pole of the thyroid and travels medially along the inferior constrictor muscle (see the image below).

Right lateral view of the superior and recurrent laryngeal nerve with the thyroid lamina removed. 

The external branch is closely associated with the superior thyroid vascular pedicle at the capsule of the superior pole of the thyroid. The external branch of the SLN enters the cricothyroid muscle laterally on its deep surface. In approximately 20% of individuals, the external branch is under the inferior constrictor muscle and cannot be visualized, but it can be stimulated using a nerve probe.^[1] 

Cernea et al^[2] developed a system to describe the level at which the external branch of the SLN crosses behind the superior thyroidal artery. Cernea type 1 nerves cross greater than 1 cm above the upper border of the thyroid gland. Type 2a nerves cross within 1 cm of the upper border of the thyroid, and type 2b nerves cross below the upper border of the gland. The external branch of the SLN is vulnerable to injury during thyroidectomy as the surgeon dissects and ligates the superior thyroid vascular pedicle, especially for Cernea type 2b nerves.

Several cadaveric studies have shown that the nerve runs below the superior border of the thyroid gland in 40% of cases, with recent clinical studies confirm‎ing that the nerves most commonly course in this inferior, Cernea type 2b, position. Thyroids that are larger than 50 grams or those with larger dimensions are correlated with nerves that run below the superior border of the thyroid gland.^[3] Injury to the external branch of the SLN may lead to vocal fatigue and the inability to perform high-energy phonation from denervation of the cricothyroid muscle.

The external branch of the SLN also contributes innervations to the pharyngeal plexus. The pharyngeal plexus, supplying the palate and pharynx, is formed by branches from the external laryngeal and pharyngeal nerves, as well as branches from the cranial nerve IX and the sympathetic trunk. 

Recurrent laryngeal nerve

The recurrent laryngeal nerve (RLN) takes a different path than the SLN on the right and left sides (see the following image).

Anatomy of the superior laryngeal nerve (SLN) recurrent laryngeal nerve (RLN), posterior view. 

The right RLN branches from the right vagus nerve in the neck at the left of T1-T2 or more inferiorly, anterior to the right subclavian artery.^[4] It travels inferior and posterior to the subclavian artery to ascend in the neck between the trachea and the esophagus, behind the right common carotid artery in the tracheoesophageal fascia. The left RLN branches from the left vagus nerve in the thorax. It travels inferior and then posterior to the arch of the aorta to ascend into the neck in between the trachea and esophagus. The terminal portion of both RLNs pass superiorly, deep to the inferior border of the inferior pharyngeal constrictor muscle, just posterior to the cricothyroid joint to supply the interarytenoid, posterior cricoarytenoid, and lateral cricoarytenoid muscles (see the images below).

Right lateral view of the superior and recurrent laryngeal nerves. 

Right lateral view of the superior and recurrent laryngeal nerve with the thyroid lamina removed. 

During anterior cervical diskectomy and fusion operations, the RLN can be exposed, causing traction or crush injuries, with postoperative dysphagia or hoarseness. Haller et al describe that greater than 80% of right RLN entered the larynx at or inferior to C6-C7, while the left RLN was invested in the tracheoesophageal fascia starting inferior to the T2 level and entered the larynx at or inferior to C6-C7.^[4] 

Most of the right RLNs course between 15-45º when entering the cricothyroid joint, whereas most of the left-sided nerves course between 0-30º.^[5] This difference is due to the more transverse path the right RLN takes when ascending in the neck. 

The RLN contains motor fibers to innervate all the intrinsic muscles of the larynx—except for the cricothyroid muscle—as well as both sensory and secretomotor fibers to the glottis, subglottis, and trachea. The terminal portion of the RLN accompanies the laryngeal branch of the inferior thyroid artery to enter the larynx after passing deep to the lower border of the inferior pharyngeal constrictor muscle, immediately behind the cricothyroid joint. 

The RLN is often described by position in relation to the branches of the inferior thyroid artery. Campos et al reviews how the relationship varies from one side to the other and among different races. They found the right RLN was most frequently found between branches of the inferior thyroid artery, followed by, in decreasing order of frequency, positions anterior and than posterior to the artery. On the left, the RLN was also placed more frequently between the branches of the inferior thyroid artery, followed by, in decreasing order of frequency, positions posterior and anterior to the inferior thyroid artery. In greater than 60% of the cases, the relationship found on one side did not occur again on the opposing side.^[6] 

Extralaryngeal RLN branching

The RLN often branches before entering the larynx. These branches may provide sensory or autonomic fibers to the larynx, trachea, esophagus, or inferior constrictor muscle. These branches may also provide motor fibers to innervate the intrinsic muscles of the larynx. Most of the branching occur superior to the intersection of the RLN and the inferior thyroid artery. The extralaryngeal branches that provide motor innervation to the larynx are often in the Berry ligament. Studies have shown that much variability to the extralaryngeal and intralaryngeal branching exists from person to person, as well as from side to side.^{[7, 8, 9, 10]} Additionally, the interarytenoid muscle is the only intrinsic laryngeal muscle with bilateral innervation. 

Traditionally, the extralaryngeal branches were described as functionally discrete fibers, separated into the anterior and posterior branches, where the anterior branches solely innervate the adductor muscles (thyroarytenoid, interarytenoid, and lateral cricoarytenoid), whereas the posterior branches innervate the abductor muscles (posterior cricoarytenoid) (see the image below). However, other studies have described no consistent functional pattern of branching of the anterior and posterior laryngeal branches.^[11] 

Right lateral view of the superior and recurrent laryngeal nerve with the thyroid lamina removed. 

The Galen anastomosis (also called the ramus anastomoticus or Ansa of Galen) is a connection between the RLN and the internal branch of the SLN (see the image above).^[7] Generally, the posterior branch of the RLN contributes to the anastomosis; however, the anterior branch can also contribute to the anastomosis.^[11] Traditionally, the Galen anastomosis has been described to provide purely sensory and autonomic innervation. More recent studies have shown that the anastomosis may also contain motor fibers.^[7] Despite being initially described as a single nerve, it may exist as a single trunk, several branches, or a plexus.

The "human communicating nerve" is an anastomosis between the external branch of the SLN and the distal RLN. Approximately 70% of human larynges have this anastomosis.^[7] The human communicating nerve may contain both sensory innervation to the larynx and motor innervation to the thyroarytenoid muscle. 

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